Which term of the ap 51,47,43 ......is a cube of itself?
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iven , AP = 51 ,47,43, 39 AND Sn=330
TO FIND - n=?
We know that
Sn =n/2(2a + (n-1)d )
Putting a=51, d=(47-51 )= -4 and Sn =330
330= n/2 (2*51 +(n-1)(-4) )
330 = n/2*2 (51+(n-1)(-2))
330 = n(51-2n +2 )
330=n(53 -2n)
-2n2 +53n-330=0
2n2 -53n+330 =0
2n2 -20n -33n +330 =0
2n(n-10 )-33(n-10 )=0
(n-10 )(2n-33)=0
Clearly n=10 as n cannot be 33/2
Therefore c)10 is right answer
HOPE IT HELP YOU
TO FIND - n=?
We know that
Sn =n/2(2a + (n-1)d )
Putting a=51, d=(47-51 )= -4 and Sn =330
330= n/2 (2*51 +(n-1)(-4) )
330 = n/2*2 (51+(n-1)(-2))
330 = n(51-2n +2 )
330=n(53 -2n)
-2n2 +53n-330=0
2n2 -53n+330 =0
2n2 -20n -33n +330 =0
2n(n-10 )-33(n-10 )=0
(n-10 )(2n-33)=0
Clearly n=10 as n cannot be 33/2
Therefore c)10 is right answer
HOPE IT HELP YOU
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